Generally, CRTs and transmissive liquid crystal displays with back lights are used in the display devices of computers and mobile devices. The displays of this type are the so-called emissive displays.
Based on recent studies, it is proposed to preferably use non-emissive reflective display devices in terms of work efficiency and wear in reading displays of texts, etc. The reflective display device, which requires no internal emission means and uses natural light, etc. for display, is good for eyes and effective to decrease the electric power consumption.
To realize further lower electric power consumption, a display device having the memory to retain displays even when the source power is turned off is expected.
As such display device is proposed a display device using a cholesteric liquid crystal. One typical cholesteric liquid crystal is chiral nematic liquid crystal. Chiral nematic liquid crystal is a liquid crystal comprising a nematic liquid crystal and a chiral reagent added to the nematic liquid crystal. Cholesteric liquid crystals typically represented by chiral nematic liquid crystal have a characteristic of reflecting selectively light of specific wavelengths.
The structure of the display device using chiral nematic liquid crystal will be explained with reference to FIG. 9. FIG. 9 is a diagrammatic view of the display device using chiral nematic liquid crystal.
As shown FIG. 9, an electrode of ITO (Indium-Tin-Oxide) is formed on a substrate 100 of glass. A photoabsobing layer 101 is formed on the back side of the substrate 100.
A substrate 104 of glass is disposed above the substrate 100 with the electrode 102 formed on, opposed to the substrate 100. An electrode 106 of ITO is formed on the side of the substrate 104, which is opposed to the electrode 102.
A liquid crystal layer 108 of chiral nematic liquid is formed between the substrates 100, 104 opposed to each other. The outer periphery of the liquid crystal layer 108 between the substrates 100, 104 is sealed with a seal compound 110 for preventing the leakage of the liquid crystal of the liquid crystal layer 108.
A display device using chiral nematic liquid crystal is disclosed in, e.g., Japanese Translation of PCT International Application No. Hei 06-507505 (1994).
Chiral nematic liquid crystal can be changed, by the application of voltages, etc., between the planer state, in which chiral nematic liquid crystal reflects that of incident light, which has a specific wavelength, and the focal conic state, in which chiral nematic liquid crystal transmits incident light.
FIG. 10A shows the planer state of chiral namatic liquid crystal. The helical axes of the liquid crystal molecules are perpendicular to the electrodes 102, 106.
In the planer state, light of a wavelength corresponding to a helical pitch of the liquid crystal molecules is reflected. The reflected wavelength can be set at a prescribed value by suitably setting an amount of the chiral reagent to be added to nematic liquid crystal to thereby change the helical axes of the liquid crystal molecules.
A wavelength λ which gives a maximum reflectance spectrum is given by the following formulaλ=n·pwherein an average refractive index of the liquid crystal is represented by n, and a helical pitch of the liquid crystal molecules is represented by p.
It is known that a band Δλ of the reflected light is larger as the refractive index anisotropy Δn of the liquid crystal is higher.
FIG. 10B shows the focal conic state of chiral nematic liquid crystal. The helical axes of the liquid crystal molecules 112 are parallel with the electrodes 102, 106.
Such arrangements of the liquid crystal are controlled to change by the application of voltages, whereby the reflective display device can be realized.
The planer state and the focal conic state are retained substantially permanently as long as no external force is applied. Accordingly, the use of chiral nematic liquid crystal makes it possible to provide a display device which can have memory of retaining display contents even when the source power is turned off.
As described above, chiral nematic liquid crystal can form reflective display devices and can retain display contents even when the source power is turned off, whereby the display devices of chiral nematic liquid crystal are noted as the next generation display device.
The method for driving the display device using the above-described chiral nematic liquid crystal will be explained with reference to FIGS. 9 to 11. FIG. 11 is views explaining the method for driving the display device using chiral nematic liquid crystal.
When a voltage is applied between the electrodes 102, 106 of the display device, the liquid crystal molecules of the liquid crystal layer 108 have a characteristic alignment corresponding to a strength of an electric field generated between the electrodes 102, 106, etc.
As shown in FIG. 11A, when a strong electric field is applied between the electrodes 102, 106, the helical structure of the liquid crystal molecules of the liquid crystal layer 108 is completely undone into homeotropic state, in which the liquid crystal molecules are aligned in the direction of the electric field. In homeotropic state, incident light on the liquid crystal layer 108 is transmitted without being reflected on the liquid crystal molecules of the liquid crystal layer 108. The incident light transmitted by the liquid crystal layer 108 is absorbed by the photoabsorbing layer 101 formed on the back side of the substrate 100.
Then, as shown in FIG. 11A, in the homeotropic state, when the electric field between the electrodes 102, 106 is abruptly removed, the helical axes of the liquid crystal molecules become perpendicular to the electrodes 102, 106. Resultantly, the chiral nematic liquid crystal becomes the planer state, which reflects selectively that of the incident light, which has a specific wavelength corresponding to a helical pitch of the liquid crystal molecules (see FIG. 10A).
On the other hand, as shown in FIG. 11B, when a weak electric field which can only undo the helical axes of the liquid crystal molecules is applied between the electrodes 102, 106 and removed, the helical axes of the liquid crystal molecules become parallel with the electrodes 102, 106. Resultantly, the chiral nematic liquid crystal becomes the focal conic state, which transmits the incident light (see FIG. 10B).
Similarly, as shown in FIG. 11C, when a strong electric filed is applied between the electrodes and gradually removed, the chiral nematic liquid crystal becomes the focal conic state, which transmits the incident light (see FIG. 10B).
When an electric field of a middle strength is applied between the electrodes 102, 106 and abruptly removed, the liquid crystal becomes a state mixing the planer state and the focal conic state, and displays of halftone can be made.
To form a reflective display device using chiral nematic liquid, the orientation state of the liquid crystal must be controlled to change to the planer state or to the focal conic state. To control the orientation state of the liquid crystal, as described above, it is necessary to apply a high voltage between the electrodes and abruptly remove the applied voltage. Accordingly, an expensive electric power source must be used.
Accordingly, a device and a method which can change the orientation state of chiral nematic liquid crystal even with an inexpensive electric power source have been required.